Muscles of Respiration
Silent (Quiet) Respiration
During quiet breathing, the diaphragm is the primary muscle of inspiration, working in concert with the parasternal intercostal muscles and scalene muscles to expand the upper rib cage. 1, 2
Inspiratory Muscles in Quiet Breathing:
- Diaphragm: The dominant muscle, responsible for most tidal volume generation
- Parasternal intercostal muscles: Play a predominant role in stabilizing and expanding the upper rib cage 2
- Scalene muscles: Act with the parasternal intercostals to expand the upper rib cage and prevent inward collapse from diaphragmatic action 1
- External intercostal muscles: Minimal activity during quiet breathing, serving as a reserve system 2
Expiratory Muscles in Quiet Breathing:
- Passive process: Expiration during quiet breathing is predominantly passive, relying on elastic recoil of the lungs and chest wall 3
- Abdominal muscles: Most subjects show tonic or phasic abdominal muscle contraction while standing and sitting (but not supine) during quiet breathing 4
Important caveat: If you can palpate phasic contraction of scalenes, sternocleidomastoids, pectoral muscles, or abdominal muscles during quiet breathing, respiratory motor output is abnormally elevated, suggesting respiratory muscle weakness or increased respiratory demand 1
Forceful Respiration
During forceful or augmented ventilation, the external intercostal muscles are recruited as a reserve system, and expiratory muscles of the chest wall and abdomen actively participate. 3, 2
Inspiratory Muscles in Forceful Breathing:
- Diaphragm: Continues as primary muscle with increased force generation
- External intercostal muscles: Significantly recruited during increased ventilation, CO2 stimulation, or inspiratory loading 2
- Parasternal intercostal muscles: Increased activity
- Scalene muscles: Enhanced recruitment
- Accessory muscles: Sternocleidomastoid and pectoral muscles become active 1
Expiratory Muscles in Forceful Breathing:
- Internal intercostal muscles: Active during forced expiration 2
- Abdominal muscles (rectus abdominis, external oblique, internal oblique/transversus abdominis): Critical for forced expiration, coughing, and expiration below functional residual capacity 3, 4
- Triangularis sterni: Important expiratory muscle during forced expiration, speech, laughing, and coughing 2
Key Physiological Principle:
During exercise and forceful breathing, respiratory muscles coordinate to minimize rib cage distortion and maximize compliance, ensuring efficient ventilation with minimal work 5. The recruitment pattern shifts from predominantly diaphragmatic work during quiet breathing to a coordinated activation of multiple muscle groups during increased ventilatory demand 3, 6.